When a strong, stable magnetic field is needed, such as in MRI or NMR, a superconducting closed circuit is formed using a superconducting coil and a persistent current switch, and a current that will hardly attenuate is passed through the closed circuit to obtain a desired magnetic field.
For a persistent current switch, a superconductor is used as a current path. Typically, ON/OFF of the persistent current switch is controlled with the superconductor heated with a heater. The superconductor has a resistance of zero (i.e., ON state) when cooled to a temperature of less than or equal to its critical temperature, but becomes a normal conductor and generates resistance therein (i.e., OFF state) when heated to a temperature of greater than or equal to the critical temperature.
When a superconducting magnet is excited, a persistent current switch is switched OFF to allow almost all portions of current supplied from an excitation power supply to pass through the superconducting magnet. In order to increase the switching speed of the persistent current switch or suppress the amount of evaporation of refrigerant during the switching time, the difference between the temperature of the persistent current switch in an ON state and the critical temperature is preferably small. Meanwhile, if the temperature of the persistent current switch in an ON state is set close to the critical temperature of the superconductor, the temperature of the superconductor becomes close to the critical temperature and the superconductor thus becomes likely to quench when an external disturbance is applied to the persistent current switch. Thus, the stability of the persistent current switch becomes low.
A low-temperature superconductor, such as NbTi, is typically cooled using liquid helium. Thus, the temperature of a persistent current switch, which uses a low-temperature superconductor, in an ON state is set to the liquid helium temperature (about 4 K), and the temperature thereof in an OFF state is around the critical temperature (about 9 K). In such a case, the temperature of the superconductor of the persistent current switch is increased by about 5 K through heating with a heater.
With the development of high-temperature superconductors in recent years, the critical temperatures of the superconductors have increased. For example, when a persistent current switch that uses a high-temperature superconductor with a critical temperature of 90 K is used in liquid helium, the temperature of the persistent current switch needs to be increased from 4K to 90 K. Such a persistent current switch that uses a high-temperature superconductor requires, in comparison with a persistent current switch that uses a low-temperature superconductor, a greater temperature increase as well as specific heat of the switch constituent material that is higher by one digit or more. Thus, an efficient heating method is needed.
For example, Patent Literature 1 discloses a persistent current switch that uses a high-temperature superconducting film, and describes YBCO and the like as examples of a high-temperature superconductor.